Understanding Cosmic Magnetism

Understanding the Universe is impossible without understanding magnetic fields. They fill interstellar space, affect the evolution of galaxies and galaxy clusters, contribute significantly to the total pressure of interstellar gas, are essential for the onset of star formation, and control the density and distribution of cosmic rays in the interstellar medium (ISM).

In spite of their importance, the origin of magnetic fields is still an open problem in fundamental physics and astrophysics. Did significant primordial fields exist before the first stars and galaxies? If not, when and how were magnetic fields subsequently generated? What maintains the present-day magnetic fields of galaxies, stars and planets?

The Faraday rotation in the Andromeda galaxy (M31) has a negative sign on the northeastern side (on the left in the image) but is positive on the opposite side. This proves that the magnetic field in M31 is highly ordered and forms a ring, pointing away from us in the northeast and towards us on the southwest side. This demonstrates the capacity of Faraday rotation to detect fields and determine their strength and direction. The SKA will be able to apply this technique out to high redshifts, encompassing millions of galaxies and even the intergalactic medium.

The most powerful probes of astrophysical magnetic fields are radio waves.

Synchrotron emission measures the field strength, while its polarisation yields the field’s orientation in the sky plane and also gives the field’s degree of ordering. Faraday rotation yields a full three-dimensional view by providing information on the field component along the line of sight, while the Zeeman effect provides an independent measure of field strength in cold gas clouds.

However, measuring cosmic magnetic fields is a difficult topic still in its infancy, restricted to nearby or bright objects.

Through the unique sensitivity and resolution of the Square Kilometre Array (SKA), the window to The Magnetic Universe can finally be fully opened. Apart from the questions we can pose today, it is important to bear in mind that the SKA will certainly discover new magnetic phenomena beyond what we can currently predict or even imagine.

An Aitoff projection of the celestial sphere in Galactic coordinates, showing recently compiled sample of 1203 rotation measures (RMs). Closed circles represent positive RMs, while open circles correspond to negative RMs, in both cases the diameter of a circle proportional to the magnitude of its RM. The 887 blue sources represent RMs toward extragalactic sources, while the 316 red sources indicate RMs of radio pulsars. The SKA will be able to measure in excess of ten million RMs, spaced at less than an arcminute between sources. Figure courtesy of Jo-Anne Brown.

For the Milky Way and for nearby galaxies and clusters, high-sensitivity mapping with the SKA of polarised synchrotron emission, combined with determinations of rotation measures (RM) for extended emission, for pulsars and for the background RM grid mentioned above will allow us to derive detailed three-dimensional maps of the strength, structure and turbulent properties of the magnetic field in these sources, which can be compared carefully with the predictions of various models for magnetic field generation.

At intermediate redshifts, polarised emission from galaxies will often be too faint to detect directly, but the magnetic fields of these sources can be traced by the RMs they produce in the polarised background grid.

This will allow detailed studies of the magnetic field configuration of individual objects at earlier epochs; comparison with studies of local galaxies will allow us to understand how magnetised structures evolve and amplify as galaxies mature.

Furthermore, from a statistical standpoint, the large number of RMs obtained from intervening galaxies and Ly-alpha absorbers will allow us to distinguish between competing models for galaxy and magnetic field evolution as a function of redshift.

At yet higher redshifts, we will take advantage of the sensitivity of the deepest SKA fields, in which we expect to detect the synchrotron emission from the youngest galaxies and proto-galaxies. RMs of the most distant polarised objects (e.g., gamma-ray bursts and quasars beyond the epoch of re-ionisation) can constrain magnetic field strengths at the earliest epoch of galaxy formation, and help distinguish between primordial and seed origins for present-day magnetic fields. Using the unique sensitivity of the SKA, it may even be feasible to measure Faraday rotation against the Cosmic Microwave Background produced by primordial magnetic fields.

The main platform on which the SKA’s studies of cosmic magnetism will be based will be an All-Sky SKA Rotation Measure Survey, in which a year of observing time will yield Faraday rotation measures (RMs) for compact polarized extragalactic sources, an increase by five orders of magnitude over current data sets, and by three orders of magnitude over what could be accomplished with the Extended Very Large Array (EVLA).

This data-set will provide an all-sky grid of RMs at a spacing of just 20–30 arcsec between sources; many these sources will have redshifts from the Sloan Digital Sky Survey (SDSS) and its successors.

This RM grid will be a powerful probe for studying foreground magnetic fields at all redshifts.

Fundamental to all these issues is the search for magnetic fields in the intergalactic medium (IGM). All empty space may be magnetised, either by outflows from galaxies, by relic lobes of radio galaxies,
or as part of the cosmic web structure. Such a field has not yet been detected, but its role as the likely seed field for galaxies and clusters, plus the prospect that the IGM field might trace and regulate structure formation in the early Universe, places considerable importance on its discovery.

This all-pervading cosmic magnetic field can finally be identified through the all-sky RM survey proposed above. Just as the correlation function of galaxies yields the power spectrum of matter, the analogous correlation function of this RM distribution can then provide the magnetic power spectrum of the IGM as a function of cosmic epoch and over a wide range of spatial scales. Such measurements will allow us to develop a detailed model of the magnetic field geometry of the IGM and of the overall Universe.

In summary, the sheer weight of RM statistics from the SKA, combined with deep polarimetric observations of individual sources, will allow us to characterize the geometry and evolution of magnetic fields in galaxies, clusters and the IGM from high redshifts through to the present, to determine whether there is a connection between the formation of magnetic fields and the formation of structure in the early Universe, and to provide solid constraints on when and how the first magnetic fields in the Universe were generated.